LONDON, England (CNN) -- More than 50 people were killed in a series of terrorist bombings on London's transport system, police said Friday as they warned more attacks were possible.
Metropolitan Police Commissioner Ian Blair said police were having "great difficulty" determining the number of deaths because of the damage at the four blast sites.
One train "still contains a number of bodies that have not been retrieved," Blair told reporters.
Police expect the death toll from Thursday's bombings to rise, but not above 100, Blair said.
Four terrorist blasts on three subway trains and a double-decker bus also injured 700 people, and a massive manhunt was under way for those responsible for London's bloodiest day since World War II.
Blair said the attacks had "all the hallmarks of al Qaeda," although he said there was "absolutely nothing to suggest" they were suicide bombings.
Half of the 700 wounded were treated on the scene and the rest were taken to hospitals, Blair said. One person died after reaching hospital. Blair said 100 people remained in hospitals overnight, and 22 had serious injuries.
He said the government was taking seriously a claim of responsibility on the Internet from a group calling itself "The Group of al Qaeda of Jihad Organization in Europe." The authenticity of the claim could not be verified.